As a promising approach for carbon dioxide capture, chemical looping combustion has been extensively investigated for more than two decades. However, the chemical looping strategy can be and has been ...extended well beyond carbon capture. In fact, significant impacts on emission reduction, energy conservation, and value-creation can be anticipated from chemical looping beyond combustion (CLBC). This article aims to demonstrate the versatility and transformational benefits of CLBC. Specifically, we focus on the use of oxygen carriers or redox catalysts for chemical production – a $4 trillion industry that consumes 40.9 quadrillion BTU of energy. Compared to state-of-the-art chemical production technologies, we illustrate that chemical looping offers significant opportunities for process intensification and exergy loss minimization. In many cases, an order of magnitude reduction in energy consumption and CO 2 emission can be realized without the needs for carbon dioxide capture. In addition to providing various CLBC examples, this article elaborates on generalized design principles for CLBC, potential benefits and pitfalls, as well as redox catalyst selection, design, optimization, and redox reaction mechanism.
Abstract
Metallic tungsten disulfide (WS
2
) monolayers have been demonstrated as promising electrocatalysts for hydrogen evolution reaction (HER) induced by the high intrinsic conductivity, however, ...the key challenges to maximize the catalytic activity are achieving the metallic WS
2
with high concentration and increasing the density of the active sites. In this work, single-atom-V catalysts (V SACs) substitutions in 1T-WS
2
monolayers (91% phase purity) are fabricated to significantly enhance the HER performance via a one-step chemical vapor deposition strategy. Atomic-resolution scanning transmission electron microscopy (STEM) imaging together with Raman spectroscopy confirm the atomic dispersion of V species on the 1T-WS
2
monolayers instead of energetically favorable 2H-WS
2
monolayers. The growth mechanism of V SACs@1T-WS
2
monolayers is experimentally and theoretically demonstrated. Density functional theory (DFT) calculations demonstrate that the activated V-atom sites play vital important role in enhancing the HER activity. In this work, it opens a novel path to directly synthesize atomically dispersed single-metal catalysts on metastable materials as efficient and robust electrocatalysts.
Ethylene is an important building block in the chemical industry; state of the art ethylene production (steam cracking) has multiple drawbacks, including high energy consumption, coke formation, and ...significant CO2 and NO x emissions. We propose a chemical looping oxidative dehydrogenation (CL-ODH) process to convert ethane into ethylene in a two-step, cyclic redox scheme. In this process, lattice oxygen in a metal oxide based redox catalyst is used to combust the hydrogen formed in ethane dehydrogenation, thereby enhancing ethylene formation while retarding coke formation. The oxygen-deprived redox catalyst is subsequently regenerated with air, releasing heat to balance the overall heat requirement. CL-ODH can realize a reduction of over 80% in primary energy consumption and pollutant emissions. The key to this process is an efficient redox catalyst with high selectivity and facile oxygen transport. Previously we determined that oxides with an Mg6MnO8 structure allow high lattice oxygen mobility and satisfactory oxygen-carrying capacity for the proposed redox reactions. However, unpromoted Mg6MnO8 exhibits poor ethylene selectivity, producing primarily CO2. In the current study, we examine the effects of various sodium-containing promoters on Mg6MnO8 CL-ODH activity and mechanism. Sodium tungstate promoted Mg6MnO8 was the most effective redox catalyst, showing an ethylene selectivity of 89.2% and yield of 68.2%, a significant improvement of thermal cracking (38.9% yield). Temperature-programmed reaction (TPR) experiments indicate that the reaction proceeds via gas-phase ethane thermal cracking in parallel with selective hydrogen combustion on the redox catalyst surface. XPS analysis indicates that the decreased ethane/ethylene oxidation activity on the sodium tungstate promoted redox catalysts results from the suppression of near-surface Mn4+. This is due to a combination of decreased surface manganese content and reduction in average Mn oxidation state. The suppression of Mn4+ results in a decrease in electrophilic surface oxygen species, inhibition of ethylene combustion, and enhanced ethylene yield.
The concept of chemical looping reactions has been widely applied in chemical industries, for example, the production of hydrogen peroxide (H2O2) from hydrogen and oxygen using 9,10-anthraquinone as ...the looping intermediate. Fundamental research on chemical looping reactions has also been applied to energy systems, for example, the splitting of water (H2O) to produce oxygen and hydrogen using ZnO as the looping intermediate. Fossil fuel chemical looping applications had been used commercially with the steam-iron process for coal from the 1900s to the 1940s and had been demonstrated at a pilot scale with the carbon dioxide acceptor process in the 1960s and 1970s. There are presently no chemical looping processes using fossil fuels in commercial operation. A key factor that hampered the continued use of these earlier processes for fossil energy operation was the inadequacy of the reactivity and recyclability of the looping particles. This factor led to higher costs for product generation using the chemical looping processes, compared to the other processes that use particularly petroleum or natural gas as feedstock. With CO2 emission control now being considered as a requirement, interest in chemical looping technology has resurfaced. In particular, chemical looping processes are appealing because of their unique ability to generate a sequestration-ready CO2 stream while yielding high energy conversion efficiency. Renewed fundamental and applied research since the early 1980s has emphasized on improvements over earlier shortcomings. New techniques have been developed for direct possessing of coal or other solid carbonaceous feedstock in chemical looping reactors. Significant progress demonstrated by the operation of several small pilot scale units worldwide indicates that the chemical looping technology may become commercially viable in the future for processing carbonaceous fuels. This perspective article describes the fundamental and applied aspects of modern chemical looping technology that utilizes fossil fuel as feedstock. It discusses chemical looping reaction thermodynamics, looping particle selection, reactor design, and process configurations. It highlights both the chemical looping combustion and the chemical looping gasification processes that are at various stages of the development. Opportunities and challenges for chemical looping process commercialization are also illustrated.
Oxygen is a critical industrial gas whose global market is projected to reach $48 billion/year within this decade. However, oxygen production is highly energy-intensive because of the limited ...efficiency of the commercial cryogenic air separation technology. The present study systematically investigated a chemical looping air separation (CLAS) approach as an alternative to cryogenic distillation. In particular, a Sr0.8Ca0.2Fe0.4Co0.6O3−δ (SCFC) oxygen sorbent was used as the basis for both experimental and simulation studies. To demonstrate the sorbent robustness, experimental studies were carried out over 10,000 redox cycles in a bench-scale testbed. Excellent sorbent stability and >90% oxygen purity were achieved using steam as the purge gas. Oxygen purity can be further increased to >95% by optimizing the operating conditions and pressure swing absorption cycle structure. Based on the experimental results, a CLAS system design and a process model were established. The process model estimates a base case CLAS energy consumption of 0.66 MJ/kg O2. This represents a 15% decrease compared to cryogenic air separation (0.78 MJ/kg O2). It is noted that most of the thermal energy consumed by CLAS is at relatively low temperatures (∼120 °C). When accounting for the quality of this low-grade heat, an energy consumption as low as 0.40 MJ/kg O2 can be anticipated for a practical system. Sensitivity analysis was also performed on the various CLAS operational and design parameters such as reactor sizes, pressure drop, thermodynamic driving forces, oxygen uptake and release rates, heat loss, and the energy consumption for steam generation. It was determined that CLAS has excellent potential to be an efficient oxygen production technology. This study also highlights the importance of developing advanced sorbents with suitable redox thermodynamics and fast redox kinetics for improved efficiency and smaller reactor sizes.
Chemical‐looping air separation has numerous potential benefits in terms of energy saving and emission reductions. The current study details a combination of density functional theory calculation and ...experimental efforts to design A‐ and B‐site codoped SrFeO3 perovskites as “low‐temperature” oxygen sorbents for chemical‐looping air separation. Substitution of the SrFeO3 host structure with Ca and Co lowers oxygen vacancy formation energy by 0.24–0.46 eV and decreases the oxygen release temperature. As a result, Sr1−xCaxFe1−yCoyO3 (SCFC; x=0.2, 0.0<y<1.0) spontaneously releases oxygen at 400–500 °C even under a relatively high oxygen partial pressure (e.g. PO2
=0.05 atm). Sr0.8Ca0.2Fe0.4Co0.6O3 exhibits a significantly higher oxygen capacity of 1.2 wt % at 400 °C and under a PO2
swing between 0.05 and 0.2 atm, when compared to the <0.2 wt % capacity for undoped a SrFeO3 (SF) and Ca‐doped Sr0.8Ca0.2FeO3 (SCF). Electrical conductivity relaxation (ECR) study demonstrates that codoping of Ca and Co lowers the activation energy of oxygen diffusion and surface oxygen exchange by 26.6 or 137.9 kJ mol−1, respectively, resulting in faster redox kinetics for SCFC than for SCF perovskite. The SCFC oxygen sorbent also exhibits excellent stability for 2000 redox cycles for air separation.
Different CLAS: A‐ and B‐site codoping effectively promotes SrFeO3‐based oxygen sorbents for low temperature chemical‐looping air separation. Codoping of Ca and Co not only reduces oxygen vacancy formation energy but also lowers energy barrier for oxygen diffusion and surface oxygen exchange, leading to >1 wt % oxygen capacity at 400 °C and fast redox kinetics.
Selective oxidation of ammonia to nitric oxide over platinum-group metal alloy gauzes is the crucial step for nitric acid production, a century-old yet greenhouse gas and capital intensive process. ...Therefore, developing alternative ammonia oxidation technologies with low environmental impacts and reduced catalyst cost are of significant importance. Herein, we propose and demonstrate a chemical looping ammonia oxidation catalyst and process to replace the costly noble metal catalysts and to reduce greenhouse gas emission. The proposed process exhibit near complete NH
conversion and exceptional NO selectivity with negligible N
O production, using nonprecious V
O
redox catalyst at 650
C. Operando spectroscopy techniques and density functional theory calculations point towards a modified, temporally separated Mars-van Krevelen mechanism featuring a reversible V
/V
redox cycle. The V = O sites are suggested to be the catalytically active center leading to the formation of the oxidation products. Meanwhile, both V = O and doubly coordinated oxygen participate in the hydrogen transfer process. The outstanding performance originates from the low activation energies for the successive hydrogen abstraction, facile NO formation as well as the easy regeneration of V = O species. Our results highlight a transformational process in extending the chemical looping strategy to producing base chemicals in a sustainable and cost-effective manner.
Biomass is a clean and renewable energy source. The efficiency for biomass conversion using conventional fuel conversion techniques, however, is constrained by the relatively low energy density and ...high moisture content of biomass. This study presents the biomass direct chemical looping (BDCL) process, an alternative process, which has the potential to thermochemically convert biomass to hydrogen and/or electricity with high efficiency. Process simulation and analysis are conducted to illustrate the individual reactor performance and the overall mass and energy management scheme of the BDCL process. A multistage model is developed based on ASPEN Plus® to account for the performance of the moving bed reactors considering the reaction equilibriums. The optimum operating conditions for the reactors are also determined. Process simulation utilizing ASPEN Plus® is then performed based on the reactor performance data obtained from the multistage model. The simulation results indicate that the BDCL process is significantly more efficient than conventional biomass conversion processes. Moreover, concentrated CO
2, produced from the BDCL process is readily sequesterable, making the process carbon negative. Several BDCL configurations are investigated for process optimization purposes. The fates of contaminants are also examined.
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